I suspect there are two reasons why English is so heard to learn:

1)It is composed of many different root languages, before we start adding modern colloquialisms: Celtic,Latin, Saxon/Viking and Norman French. Giving it roots in Celtic, Latin and Germanic languages, which I understand is fairly unusual. It can become quite intresting when you look at the roots of some usages, for example a cow is a cow in the feild where the Saxon serf looked after it, but Beef on the table where a Norman lord ate it (look at the other meats, and you can work out what was not 'noble' fair).

2) When the the printing press was invented the first person to try to write a dictionary for the language took his spellings from different areas of the country, which affected the pronouncation, hence rough is pronounced ruff, whilst although is prounounced 'all tho.

I'm English and long ago came to the conclusion that there is no rhyme or reason for the way we spell or pronounce things, but that we, and the rest of the world are stuck with it. Good luck

CFJS